There are several applications that operate based on the detection of extremely small magnetic fields, on the order of a few picotesla at DC to a few Hz frequency range. Cardiac magnetic signal detection for biomedical uses is one such application. Military installations are another application for the detection of extremely small magnetic fields. For example, an armed soldier at several hundred meters can be detected and identified by the signature magnetic field.
Giant magnetoresistance (GMR) and similar magnetic sensor elements developed for use in magnetic recording heads are very sensitive and their sensitivity can be increased by the addition of flux concentrators to create a sensor device. Magnetoresistance sensor devices are low cost because they can be mass produced by batch processing on silicon wafers and the drive and read out electronics are relatively simple. The resistance within a magnetoresistance sensor device is sensitive to the magnitude and direction of a magnetic field external to the sensor device. To detect the relative motion between the desired target and the magnetic sensor generally requires high sensitivity in the frequency range below a few Hz.
Unfortunately, most magnetoresistance sensor devices tend to suffer from 1/f noise, which is common phenomena in electronic devices and sensors, where the noise level per unit of signal bandwidth is roughly proportional to the reciprocal of the frequency, which severely limits the effectiveness of noise reduction by bandwidth reduction at very low frequencies. Furthermore, there is generally a tendency for those sensor devices that have a more sensitive response to magnetic fields to also have more 1/f noise. Magnetization fluctuations in magnetoresistive heads tend to be a fundamental limit on their signal-to-noise ratio. The 1/f noise of the sensor devices themselves prevents obtaining the desired sensitivity for the possible bandwidth, as the noise at 1 Hz is several orders of magnitude greater than at several KHz.
Various attempts have been made to reduce the sensor 1/f noise by shifting the DC magnetic signal to the frequency of a mechanical oscillation of the flux concentrator spacing. Although this does reduce the 1/f noise from the sensor element, the noise from the flux concentrators remains and is not reduced. Furthermore, applying the motion to the flux concentrators makes it impractical to use large flux concentrators which would have a larger concentration ratio than smaller sized concentrators.